Method of converting unsaturated organic compounds to organoboron compounds



METHOD OF CONVERTING UNSATURATED ORGANIC COMPOUNDS TO .ORGANOBO- RON COMPOUNDS I Herbert C. Brown, West Lafayette, Ind. No Drawing. Application February l, 1957 Serial No. 637,615

' 24 Claims. (Cl. 260-6065) This invention relates to the preparation of organoboron compounds and more particularly to the preparation of organoboron compounds having the formula R,B in which R is a saturated hydrocarbon radical, such as an alkyl radical.

Meerwein, Hinz, Majert and Sonke (I. Pruitt-Chem, 147, 240 (1936)) have described the synthesis oftrialkylborons. Thus, triethylboron has been prepared "by the reaction of ethylmagnesium bromide on boron triiiuoride in diethyl ether as solvent. The reaction is illustrated by the equations: p

C H Br+Mg C H MgBr This procedure has disadvantages since it requires a number of intermediates, suchyas ethyl bromide, magnesium and boron trifluoride, and involves the'formation of a byproduct, such as MgBrF, which must be recovered for the economic production of trialkylborons.v

Hurd (Jorn. Am. Chem. Soc., 70, 2053 (1948)) has reported that gaseous diborane may be reacted with a large excess of olefinic hydrocarbons in sealed tubesat 100 C. for extended periods of time. Thus, a mixture of triisobutylboron and tri-t-bu tylboron was obtained by the reaction of isobutylene and diborane in a sealed tube United States Patent Patented Feb. 16, 1960 a CC October 30, 1956, I have described thepreparation of organoboron compounds by reacting an unsaturated organic compound having one or more olefinicdouble bonds or an acetylenic triple bond, such as l-butene, butadiene and 3-hexyne, with a reagent consisting of an alkali metal borohydride and a halide of a polyvalent metal, such as aluminum chloride, titanium tetrachloride,

'etc., in an inert liquid carrier, such as a solvent for, the

alkali metal borohydride. This reagent did not include halides of metalloids, such as boron halides. The addition of a 'stoichiometric quantity of a boron halide to a solution of an alkali metal borohydride results in a rapid and essentially complete loss of the active hydrogen of the solution and, consequently, it might be expected that such a reagent would be unsatisfactory. 4

, l have discovered that under certain conditions a reagent consisting of 'a boron halide and an alkali metal borohydride is exceedingly effective and eflicient for the preparation of org anoboron'compounds by reaction with an unsaturated organic compound. Thus, by slowly adding a boron halide to a solution of an alkali metal borohydride containing the unsaturated organic compound, I

' it is possible to realize a 90 to 100 percent utilization of resents a saturated hydrocarbon radical:

the hydride content of the'alkali metal borohydride in the hydt boration of unsaturated compounds.

The use of 'a boron halide for this purpose has the advantage of increasing the possible yields. In utilizing a metal halide, such as .a'luminum'chloride, to-bring about the hydrob'oration of olefins and other unsaturated compounds, only percent of the available hydride of the alkali metal borohydride can be utilized. On the other hand, the use of boron-halides permits percent of the available hydride of the borohydride to be utilized. This 7 is illustrated by the following equations where repat '100 C. for 24 hours. Reaction of ethylene with diborane at 100 C. for 96 hours produced triethylboron. R..S. Brokaw and R. N. Pease (Iourn. Am. Chem. Soc.,

72, 3237 l950); ibid., 72 5263 (1950)) have reported that gaseous-olefins, such as ethylene, propylene and 1- I -butene, react with aluminum borohydride at elevated tem.*'

peratures to form trialkylborons along 'vvith mixed alkylaluminum hydrides. Thus, thereaction of ethylene with aluminum borohydride at C. is postulated tobe 4)s+ z 4- z 5)a -FCz s K di a 5 4)2+ 2 4" 2 s)a zHs)4 z z The last two of the above mentioned'methods offer some advantages over the Grignard route in that they utilize olefins instead of the more expensive alkyl halides.

However, neither diborane nor aluminum borohydride are commercially available materials at the present time.

Moreover, the characteristics of these materials are such that they'are diflicult to handlepThus, diboranc is a gas, rapidly hydrolyzed by moisture, and -very sensitive to air oxidation. Aluminum borohydride is also sensitive to water and reacts with explosive violeneewith air.'

There would be obvious advantages to the utilization of the alkali metal borohy'drides .for the synthesis of organoborons. The alkali metal borohydrides are now commercially available and exhibit remarkable stability to air and water. However, it islwell known that olefins do not react with the alkali metal borohydrides. Thus, sodium borohydride is widely used for the selective reduction of the carbonyl groups in unsaturated aldehydes organic compoundin stoichiometric proportions may beadded slowly at arate comparable to the rate of the hy- I droboration reaction to a solution or slurry of the alkali metal borohydride. Preferably, however, the boron halide is added slowly to a solution or slurry of the alkalimetal v borohydride containing about four moles of the unsaturated organic compound per mole of the alkali metal boro hydride. The reaction proceeds smoothly at room temperature. However, temperature is not a critical factor since the reaction appears to proceed equally well at temperaturesof 0 C. or 50 C.

' While in the practiceof the present invention it is preferred to employ'a solvent for the alkali metal boroand ketones. No reaction has been observed in heating olefins with the alkali 'metal borohydrides at temperatures ashighas200 o In my copending application Serial'No. 619,355, filed non-solvent liquid carrier.

alkali metal borohydride be dissolved in a suitable inert hydride as the inert liquid carrier, other inert liquid carriers may be used. Usually, however, when the liquid carrier is not, a solvent for the alkali metal boroflhydride, more satisfactory results are obtained if a solvent, preferably in an amount suflicient to solubilize the alkali metal borohydride, is used in conjunction with the It is preferable that the glycol or the dimethylether or triethylene glycol for sov dium borohydride. Sat'wfactory results may be obtained using slurries of the alkali metal borohydride, such as a slurry of lithium borohydride in a mixture of tertrahydrofuran and toluene or a slurry of sodium borohydride in a mixture of tetrahydrofuran and dimethyle'ther of diethylene glycol. Potassium borohydride also may be used but is somewhat less desirable than sodium or lithium borohydrides because of its lower solubility in desirable solvents.

Boron trifluoride is readily used in the reaction over a wide temperature range. Boron trichloride also is highly effective. However, since boron trichloride exhibits some tendency to split tetrahydrofuran and the dimethylethers of diand triethylene glycol, it is preferably used in these solvents at a temperature of 25 f C. or lower. It is quite stable in diethyl ether and can be freely used in this solvent. Boron tribromide is also effective but it is more costly than other boron halides and exhibits a tendency to split the ethers. Boron triiodide can be used but is difiicult to handle and tends to react with the-solvents.

The term unsaturated" as used herein is intended to refer to organic compounds which owe their unsaturation to the presence of one'or more carbonto carbon double bonds or carbon to carbon triple bonds. In the sense in which this term is used herein, aromatic rings, such as benzene and toluene, and alicyclic rings, such as cyclohexane, are not unsaturated and may be present in the unsaturated organic compound as inert substituents.

The method of the present invention is applicable broadly for conversion to organoboron compounds of olefins such as l-p'enten'e, l-octene and styrene, dienes such as butadiene andcyclohexadiene, and acetylenes such as l-hexyne and Z-hexyne. It also is applicable for the conversion of substituted olefins, such as methylmethacrylate, triallyl borate, m-nitrostyrene, nitroethylene, allylethylether and vinylbutyl ether. Thus, certain functional groups, such as nitro-, carbethoxy, and ether groups can be accommodated. Other groups, such as aldehyde and keto groups are reduced so that the hydroboration is accompanied by reduction of these functional groups. More especially, the invention contemplates the conversion of olefins, such as 1-pentene, 2-pentene, l-hexene,

l-octene, diisobutylene, cyclopentene, cyclohexene, styrene, a-methylstyrene and 5,;8-diphenylethylene, to organoboron compounds having the formula R B in which R represents a saturated hydrocarbon radical.

The process of the invention is remarkably simple. For example, a solution of 3 moles of sodium borohydride and 12 moles of the organic reactant in 2 liters of dimethylether of diethylene glycol is maintained at 25 to 40 C. under a nitrogen atmospherein a stirred S-liter flask. Pure boron trifluoride, 4 moles, is passed in over a period of one hour, the temperature being maintained at 25 to 40 C. through external cooling. Waterthen is added. Theorganoboron layer is separated in a nitrogen atmosphere, washed with several portions of water and then distilled under vacuum to isolate the pure trialkylboron. The yields are exceedingly good as illustrated by the following table.

Boron trifluoride sodium bor 0hydride In a dry 5-liter round-bottomed flask equipped with a mercury sealed stirrer, condenser, inlet tube and thermometer was placed 2 liters of purified dimethylether of diethylene glycol, 114 grams (3.0 moles) of sodium borohydride and 840 grams (12.0 moles) of l-pentene. A nitrogen atmosphere was maintained. Boron trifluoride, 4*moles, was passed into the reaction mixture over a period of one hour with vigorous cooling to maintain the temperature-within the flask at 3540 C. At the end of the reaction water was added to dissolve the solvent. The aqueous layer wasremoved. The product layer was washed several times further with water in a nitrogen atmosphere and the product was then distilled under reduced pressure. There was obtained 805 grams of tii-n-pentylboron, B.P. 94-95 C. 'at 2 mm., a yield of percent.

Identical procedures were used for the reaction of l-hexene, l-octene, l-diisobutylene, Z-diisobutylene, cyclopentene, cyclohexene, styrene, a-methylstyrene and 5,;8-diphenylethylene. In all cases, 80-90 percent yields of the trialkylboron were realized. v

The same procedure was used on a scale of 0.1 mole for diallyl (1,5-hexadiene), 3-hexyne, methylmethacrylate, triallylborate, m-nitrostyrene, nitroethylene, allylethyl ether and vinyl butyl ether. In these reactions the reaction product was taken up in ether, washed with water and the formation of organoboron derivative established by analysis after oxidation by alkaline hydrogen peroxide.

Boron trifluoride and lithium lmrohydride tilled. The ether was removed on the steam bath and the product then distilled under reduced pressure (nitrogen atmosphere). There was obtained an 84 percent yield of tricyclohexyl boron, B.P. -132 C. at 2 mm. 1

Similar procedures were use for butadiene, styrene, loctene and vinylbutyl ether.

Boron trichloride and sodium borohydride In a l-liter flask was placed 400 cc. of purified dimethylether of diethylene glycol, 9.5 grams of sodium borohydride and 82 grams (1.0 mole) of cyclohexene.

A nitrogen atmosphere was maintained and 0.34 mole of boron trichloride dissolved in diethyl ether was slowly added through a dropping funnel over a period of one hour. The temperature was maintained below 25 C. The reaction mixture was treated with 200 ml. of diethyl ether, treated with water and the ether layer transferred to a distillation flask under nitrogen for distillation. There was obtained an 88 percent yield of tricyclohexylat 2 mm.

This procedure also was applied for the reaction of 2- pentene, styrene, allyl chloride and pcarbethoxystyrene.

The claims of the present application are directed apecifically to the preparation of an organoboron compound by reacting an unsaturated organic compound with a reagent consisting of an alkali metal borohydride and a boron halide. The claims of my copending application Serial No. 641,272, filed February 20, 1957, are directed broadly to the reduction of a chemical compound having a reducible functional group with a reagent consisting Of a a kali metal borohydride and a boron halide.

mamas I claim:

1. In a method of preparing an organoboroncompound by reacting an unsaturated organic compound with a reagent consisting of 'an alkali metal borohydride and a boron halide, the step which comprises -slowly bringing one-component of said reagent in reactive oontact in an inert liquid carrier with the other component compound, said unsaturated organic compound being selected from the group consisting of olefins, cycloolefins,

kenes, carbalkoxyaryl alkenes, alkenyl cthers, nitroalkenes, nitroarylalkenes and alkenylborates.

2. The method as claimed by claim 1 wherein a mixture of a boron halide and the selected unsaturated organic compound in said stoichiometric proportions is added slowly. to an alkali metal horohydride in an inert liquid carrier until a substantially stoichiometric amount of saidborohydride has reacted with said mixture to form said organoboron compound. 7

3. The method as claimed by claim 1 wherei'n'a boron halide is slowly added to a mixture of the selected unsaturated organic compound and an alkali metal borohydride in said stoichiometric proportions in an inert liquid carrier until a substantially stoichiometric amount of said boron halide has reacted with said mixture to form said organoboron compound.

4. The method claimed by claim 3 whereinthe liquid carrier-includes an inert solvent for the. alkali metal borohydride'." r.

5. The method claimed by claim 4 whereinthe amount of solvent is sufiicient to solubilize the .borohydride.

6. The method claimed by claim 5 wherein the boron halide is selected from the group consisting of boron trifluoride and boron trichloride. I

7. The method claimed by claim 6 wherein-the alkali metal borohydridelis sodium borohydride.

'15 dienes, alkynes, arylalkenes, alkyl alkenoates, haloal- 8. The method claimed by claim 6 wherein the alkali metal borohydride is lithium borohydride.

9. The method claimed by claim 7 whereinthe solvent is the dimethylether of diethylene .glycol.

10. The method claimed by claim 7 whereinthe sol-,

pound'by reacting an olefin with a reagent consisting of an alkali metal borohydride and a boron halide, the step which comprises slowly bringing one component of said reagent in reactive contact'in an inert liquid'carrier' with the other component of the reagent in the presence of an amount of the olefin corresponding stoichiometrically at least to the amount of said one component .until substantially stoichiometric amounts of said olefin and said alkali metal borohydride and said boron halide have reacted to form said organoboron compound.

.14. The method as claimed byclaim 13 wherein a mixturev of a boron halide and an olefin in said stoichiometric proportions is added slowly to an alkali metal borohydride in an inert liquid carrier until a substantially stoichiometric amount of said boroliydride has reacted with said mixture to form said organoboron com.- pound.

15. The method as claimed by claim 13 wherein a boron halide is slowly added to a mixture of an olefin and an alkali metal borohydride in said stoichiometric proportions in an inert liquid carrier, until a substantially stoichiometric amount of said boron halide has ,reacted with said mixture to form said organoboron compound.

. 16. The method claimed by claim 15 wherein the liquid carrier includes an inert solvent for the alkali metal borohydride.

17. The method claimed by claim 16 wherein the amount of solvent is suficient to solubilize the borohydride.

18. The method claimed by claim 17 wherein the boron halide is selected from the group consisting of boron trifiuoride-and borontrichloride.

19. The method claimed by claim 18 wherein the alkali metal borohydride is sodium borohydride.

20. The method claimed by claim 18 wherein the alkali metal borohydride is lithium borohydride.

21. The method claimed by claim l9-wherein the solvent is the dimethyletherof dicthylene glycol.

22. The method claimed by claim 19 wherein they solvent is the dimethylether of triethylene glycol.

23. The method claimed by claim 20 wherein the solvent is diethyl ether. v

24. The method claimed by claim 20 wherein the solvent is tetrahydrofuran.

No references cited. 

1. IN A METHOD OF PREPARING AN ORGANOBORON COMPOUND BY REACTING AN UNSATURATED ORGANIC COMPOUND WITH A REAGENT CONSISTING OF AN ALKALI METAL BOROHYDRIDE AND A BORON HALIDE, THE STEP WHICH COMPRISES SLOWLY BRINGING ONE COMPONENT OF SAID REAGENT IN REACTIVE CONTACT IN AN INERT LIQUID CARRIER WITH THE OTHER COMPONENT OF THE REAGENT IN THE PRESENCE OF AN AMOUNT OF THE UNSATURATED ORGANIC COMPOJND CORRESPONDING STOICHIOMETRICALLY AT LEAST TO THE AMOUNT OF SAID ONE COMPONENT UNTIL SUBSTANTIALLY STOICHIOMETRIC AMOUNTS OF SAID ORGANIC COMPOUND AND SAIOD ALKALI METAL BOROHYDRIDE AND SAID BORON HALIDE HAVE REACTED TO FROM SAID ORGANOBORON COMPOUND, SAID UNSATURATED ORGANIC COMPOUND BEING SELECTED FROM THE GROUP CONSISTING OF OLEFINS, CYCLOOLEFINS, DIENES, ALKYNES, ARYLALKENES, ALKYL ALKENOATES, HALOALKENES, CARBALKOXYARYL ALKENES, ALKENYL ETHERS, NITROALKENES, NITROARYLALKENES AND ALKENYL BORATES. 